When fastening two or more items together with greater than or equal to two fasteners, issues of non-alignment arise. Typically, adjustment of tolerances can yield a set of parts that can be assembled. In high vibration environments (i.e., launch vehicles, high speed trains, racecars, hammer mills, steel recyclers, etc) shear joints are required and are typically designed with very tight tolerances to prevent relative motion between the parts of an assembly. In most cases, these parts must be match machined to achieve desired tolerance control. Tolerance control processes can be very expensive, particularly when match machining of large parts too very tight tolerances is involved.
FIG. 1 shows a prior art eccentric cylinder. The cylinder 102 has an outer cylindrical surface 106 and an inner cylindrical hole 104. The axes of the hole 104 and the outer surface 106 are parallel but not aligned. Therefore, the hole 104 is eccentric relative to the outer cylindrical surface 106. Rotation of the type of bushing shown in FIG. 1 does not allow alignment to a desired center.
FIG. 2 shows the eccentric elements shown in FIG. 1 as part of a prior art fastening system. The eccentric cylinder 102 allows misalignments between fastening features in a first part 202 and a second part 204 to be compensated for. Unfortunately, the range of adjustment of the position of the hole one of four relative to the parts 202, 204 is extremely limited. It should be fairly obvious that unless the misalignments between the parts 202, 204 is exactly the amount of the eccentric offset off the eccentric cylinder 102, then the axis of the hole 104 will be misaligned relative to a fastening feature of the second part 204. Thus, when a bolt is securely fastened with a nut through the assembly, it is highly likely that the bolt will experience bending stresses. Cyclic bending stress can cause fatigue, which can significantly reduce material strength properties.
FIG. 3 shows a prior art double eccentric cylindrical system. The two piece double eccentric system overcomes the misalignments issues found with a single eccentric cylindrical fastening system shown in FIGS. 1 and 2. The eccentric cylinder 102 with offset hole 104 is shown. Also shown is a second eccentric cylinder 302. The second eccentric cylinder 302 has an inner cylindrical surface 304 and an outer cylindrical surface 306. The axis of the inner cylindrical surface 304 and the axis of the outer cylindrical surface 306 are parallel, but offset. Thus, the inner and outer cylindrical surfaces 304, 306 are eccentric. The outer cylindrical surface 106 of the first eccentric cylinder 102 fits inside of the inner cylindrical surface 304 of the second eccentric cylinder 302.
The two piece double eccentric cylindrical system shown in FIG. 3 allows the location of the hole 104 to be positioned anywhere from the axis of the outer cylindrical surface 306 to a combination of the eccentric offsets away from that axis. Thus, problems with fastener bending stresses and other issues with misalignments between parts and fastening features can be greatly reduced.
Unfortunately, the prior art double eccentric cylindrical fastening system shown in FIG. 3 has several disadvantages. First of all, the machining tolerances required for both the first and second eccentric cylinders 102, 302 and that of the parts to be fastened together are quite high. It is very difficult to machine all of the features required to allow parts to be assembled while achieving a zero clearance joint.
When the assembled parts are used in a high vibration environment, the combination of excess clearances in the prior art fastening schemes can allow the parts to move relative to each other. In many instances this movement is unacceptable.
Prior art cylindrical bushings with offset axes for their outside versus inside diameters allows for rotational adjustment of the bushings to find a desired center (i.e., axis of fastener or vault hinge pin). To prevent the bushings from rotating, it is understood that the prior art relies on friction at the perpendicular faces of the bushings/fixed parts or on tack welding the bushings together and to the fixed part after completion of the alignment process. This may be tolerable for a lightly loaded static joint (i.e., a joint that does not experience cyclic or fluctuating loads).
Experience with high vibration environments has shown that friction alone can not be relied upon to hold joints together. Under high vibration/shock loads, effective fastener preload is reduced with a corresponding reduction in friction forces. As friction forces reduce, the joint begins slipping back and forth in a cyclic fashion; this can lead to fatigue and subsequent catastrophic failure (i.e., broken parts). Tack welding prevents motion under light cyclic loads. Load management can be increased with deeper weld penetrations, but it is impractical to achieve a full penetration welds with current practice. In addition, welding induces internal stresses that cause dimensional changes to the parts being welded and can alter the desired alignment of the joint. If tack welding is done incorrectly, rework of the joint can be impractical.
Thus, there is a need for a fastening system that does not require super high tolerance machining of large parts, that works well in high vibration and/or shock environments and that can provide a means of adjustment in the field.